Anaerobic Digestion: An Alternative To Composting

Hi all – I wanted to introduce this post with a quick note because it has been written just for Ground to Ground by a guest writer. It is a topic I have little experience with myself, and having reviewed the content, including videos, it is something I’m wanting to try for myself. Well it’s on the list anyway! Enjoy – Shane.

How Much Do We Throw Away?

Municipal solid waste (MSW) is what we commonly call trash, garbage, refuse, or rubbish. And we generate a lot of it. The United States Environmental Protection Agency (EPA) estimates that in 2009 Americans generated 243 million tons of MSW, or about four and a half pounds per day per person. Most of this ends up in landfill, where biodegradable material (organic waste, like food scraps) decomposes very slowly. For example, one researcher established that a piece of meat in a landfill will remain essentially intact for as long as 30 years. So, our trash just keeps piling up.

What’s In Trash And What Happens To It?

MSW consists of a variety of materials — plastic bottles, old batteries, aluminum cans, baby diapers, paper, food scraps, and so on. Paper represents the largest percentage of what we discard, about 28%.

Organic Material

Food waste is the second-largest fraction of MSW, representing about 14%. In the US, that translated to around 34 million tons of food waste in 2009, only 3% of which was recycled or recovered. Since food scraps are biodegradable, we can treat them as a valuable resource by recovering the nutrients and energy they contain. This is something we can do at home.

Composting: How We Can Put Biodegradable Waste To Better Use

Composting organic waste is an effective and useful way to reduce how much we send to landfill, as well as to recycle organic waste like food scraps and used coffee grounds. The good news is that composting has steadily become more popular, particularly among gardeners. It’s a simple process one can do at home, and it helps significantly to improve our impact on the environment we all share. But are there other alternatives?

An Alternative To Composting

Composting relies on what are called detritivores, which means “things that eat detritus”, to break down organic waste into a form we can use as a natural fertilizer. Detritivores important to the composting process are organisms like worms and bacteria. There are literally millions of types of bacteria, and many are beneficial to us in one way or another. Familiar things like composting and probiotics rely on beneficial bacteria.

Another way we can gain benefit from bacteria and improve our impact on the environment is a process called Anaerobic Digestion (AD), meaning digestion in the absence of oxygen (O2). Similar to composting, in AD bacteria consume organic waste such as food scraps, silage, and animal waste and generate an environmentally benign byproduct that can be used as a natural fertilizer. Unlike composting, however, AD also produces biogas, which consists of about 2/3 methane (CH4). Natural gas is methane with a small amount of other trace gases, so biogas can be used as fuel like natural gas.

How Can We Use Anaerobic Digestion At Home?

Anaerobic digestion is not a new process. It existed long before humans appeared on Earth, and humans have been using it in various forms for centuries. In countries like India and China, where many people either lack access to municipal utilities services or the money to pay for them, a lot of homes use simple anaerobic digester setups to both dispose of their organic waste and generate gas that can be used for cooking, heating, and water.

In countries like the UK and the US, you can easily get what you need to set up a simple, effective anaerobic digester at the local hardware store. In its most basic form, all that’s required is a 55 gallon drum or similar container to house the organic waste you want to digest, some common plumbing materials (like PVC pipe), and something to collect the gas into. Clever people have come up with several specific ways of doing AD at home.

What Will Make Your Digester Successful?

AD occurs in three broad stages: (1) hydrolysis, in which the initial digestion takes place as the result of one group of bacteria; this is analogous to what occurs in the mouth and stomach. (2) Acidogenesis, in which a second group of bacteria convert the predigested material into a weak acid, usually acetic acid (vinegar); this is roughly similar to what the small intestine does. And (3), methanogenesis, in which a group of bacteria known as methanogens convert the acid into methane, as do some organisms in the large intestine.

Keep It Anoxic

Methanogens are particularly sensitive to oxygen, meaning they die quickly when exposed to O2. Therefore, doing your best to keep your digester sealed so that it remains anoxic (free of oxygen) is important. One way you can accomplish this is by keeping a decent water level in your digester tank and not mixing during digestion so that the digesting organic material stays well below water level. You can also use an airtight valve for input (cheap and readily available at the local hardware store) that you only open when adding new material for digestion.

Keep It Warm

There are three broad temperature ranges that different types of methanogens prefer. Psychrophilic, meaning “cool loving”; mesophilic, meaning “middle loving”, and thermophilic, meaning “hot loving”. Anaerobic digestion gets faster as the temperature rises, so keeping your digester warm facilitates the process. Some people heat their digesters to favor the growth and activity of thermophilic methanogens. This is not necessary, but it does speed up the process.

What’s more important is to maintain as consistent a temperature as you can. In the Chinese Dome Digester design, the digester is buried a few feet below the ground (usually it is a small concrete dome) to take advantage of the Earth’s insulating properties. Another method is to insulate a 55 gallon drum well, which is preferably kept enclosed – similar to a “well house”. There are many ways to keep a home digester’s temperature roughly consistent.

Mash Or Grind Your Waste

Anaerobic digestion is what occurs in the latter part of an animal’s digestive system. Before it gets there, however, the animal grinds it up by chewing. Likewise, mashing your waste with water using a meshed-plunger, a garbage disposal, or even an old-fashioned hand-cranked meat grinder is beneficial. However you do it, it’s a good idea to mash the organic waste with a bit of water into a slurry (a homogenous paste). You don’t have to go overboard, but this step will aid your digester in doing its job.

Keep The Poisons Out

We use bleach, alcohol, detergents, peroxide, antibiotics, and so on to kill bacteria. These things can be present in your trash, so as with a septic tank, it’s a good idea to do your best to make sure no toxins like these get into your digester. It’s also important to try to keep metals, cleaners, and solvents out of your digester tank.

Keep The Pressure Ambient

This is relatively easy if you use an inflatable bag or the floating barrel design depicted in one of the videos below. The basic idea is to give the gas a low-pressure place to go as it is being formed so that not too much back pressure builds up in your digester.

What Can You Do With The Gas?

The biogas produced by anaerobic digestion is mostly methane (CH4), with some carbon dioxide (CO2) and trace gases. You can use it in conventional gas-burning appliances, like a stove, with some small, inexpensive modifications. It’s a good idea to scrub the gas first, which just means that you take steps to remove some of the non-combustible contaminants.

A simple way to accomplish this is to put what’s called an “iron sponge” in the gas line before it reaches your appliances, which is nothing more than an air-tight jar with steel wool in it. It’s also good practice to include a “bubbler”, which is just a small reservoir of water the gas moves through before the end use.

Something to Think About

The long and short of it is, if you are a proponent of living simply and concerned about the ever-growing problems of waste disposal, resource depletion, and pollution, setting up your own home anaerobic digester is one great way to help solve these problems. It has the added advantages of providing you with fuel you can use in gas appliances or an electrical generator, thereby saving you money, and reducing green house gases, which enables you to lower your carbon footprint.

How can this be true if you burn the gas and make CO2? Decomposition of organic waste like food scraps (or animal manure) produces methane and CO2, which is then simply released into the atmosphere. Landfill is a major source of human-related methane in the United States, accounting for more than 20 percent of all methane emissions. A molecule of methane has approximately 21 times the warming potential of a molecule of CO2, so by capturing and burning the methane, you reduce your effective contribution to Global Warming twenty fold! Who can resist that?

Eric is a biologist working in the field of Bioremediation. He has a passion for conservation and blogs over at Living Simply on topics related to sustainable, simple living.

It was a long read but worth it! Can I sugest you jump into the video at some stage and then get back to the more details parts of the article. AD seems like a fairly easy thing for most people and is a good alternative to composting.
Thanks ground to ground, Ross.